This invention relates to a method for transmitting a color video signal over a telephone line, an audio tape recorder or other narrow-band transmission lines.
In converting or reversely converting what is called the broad-band video signal for the standard broadcasting system of the NTSC Color System into a narrow-band signal, a generally well known method is by storing the video signal temporarily in a memory and changing the clock pulse frequency applied when reading the video signal, thereby converting the occupied band of the signal. In order to transmit the phase of the carrier chrominance signal faithfully in the NTSC color system, the clock frequency of the memory, i.e., the sampling frequency, is required to be approximately three times the carrier frequency thereof. In the case of an NTSC color system, the carrier of the carrier chrominance signal (hereinafter referred to as C signal) is 3.58 MHz, and therefore the clock frequency is 10.74 MHz or three times the carrier frequency. On the other hand, the luminance signal (hereinafter referred to as the Y signal) has a band of 4.5 MHz. For reducing this signal into picture elements, the clock frequency of about 9 MHz or a double frequency is sufficient in view of the limit of visibility of an ordinary color monitor.
The problems posed here are that a high-speed memory is required, that a memory capacity more than required must be provided for the Y signal band, and that in order to reduce such distortion as differential phase or differential gain, superior transmission characteristics are required of the narrow-band transmission line or, if the transmission characteristics are inferior, the transmission time is required to be lengthened.